A lanthanum chromite-based conductive sintered body is excellent in chemical stability at high temperature as well as being high in electron conductivity, and has therefore been used for a separator, an interconnector, or the like, of a solid electrolyte fuel cell.
FIG. 1 shows one example of a solid electrolyte fuel cell, and FIG. 2 shows one example of a cell stack of the solid electrolyte fuel cell. The cell stack has a structure that a plurality of fuel cells 30 are assembled in lines with a current collector 40 made of a metal felt or the like interposed between one fuel cell and another fuel cell adjacent thereto, thereby electrically connecting a fuel electrode layer 32 of one fuel cell to an oxygen electrode layer (air electrode) 34 of another fuel cell.
The fuel cell 30 is constituted by sequentially disposing the fuel electrode layer 32, a solid electrolyte layer 33, and the oxygen electrode layer 34 made of conductive ceramics, on an outer circumferential surface of a cermet-made support base 31 (which is conductive and of which inner part serves as a fuel gas passage) having flat-tube shape, and there is an interconnector 35 provided on a surface of support base 31 which is not covered by the solid electrolyte layer 33 or the oxygen electrode layer 34. As is obvious from FIG. 2, the interconnector 35 is electrically connected to the support base 31 so as not to be connected to the oxygen electrode layer 34.
The interconnector 35 is made of conductive ceramics less susceptible to fuel gas and oxygen-containing gas such as air, which conductive ceramics has to be so dense as to be a reliable block between the fuel gas flowing through the support base 31 and the fuel electrode layer 32 and the oxygen-containing gas flowing outside the oxygen electrode layer 34. In such a fuel cell, power is generated at high temperature by flowing the fuel gas (hydrogen) through a fuel gas passage 31a of the support base 31 as well as flowing the air (oxygen) through the oxygen electrode layer 34.
Further, the current collector 40, which is disposed between the fuel cells adjacent to each other, is electrically connected to the support base 31 of one fuel cell with the interconnector 35 therebetween and moreover connected to the oxygen electrode layer 34 of the other fuel cell, whereby the adjacent fuel cells are connected in series.
In the fuel cell constituting the above fuel cell apparatus, the support base 31 is mainly formed of Ni and Y2O3; the fuel electrode layer 32 is formed of ZrO2 (YSZ) containing Ni and Y2O3; the solid electrolyte layer 33 is formed of ZrO2 (YSZ) containing Y2O3; the oxygen electrode layer 34 is formed of lanthanum ferrite-based perovskite composite oxide; and the interconnector 35 is formed of lanthanum chromite-based perovskite composite oxide, with the result that the respective members can be formed by simultaneous firing. This is called a co-sintering method, and the co-sintering method requires a small number of production steps and is therefore advantageous in enhancement of yield and cost reduction in manufacturing the cell. In particular, it is known that close coefficients of thermal expansion of the respective members result in a fuel cell not suffering from cracks or flaking (refer to Japanese Unexamined Patent Publication JP-A 2004-146334, for example).
Lanthanum chromite constituting the interconnector is a poor sinterability in general and has been therefore considered to be unsuitable for co-sintering. As a solution for it, it is conventionally known that Mg or Al and Mn, Fe, Co, Ni, Znt Cu, V, and Ti are contained in B site of lanthanum chromite (refer to Japanese Unexamined Patent Publication JP-A 8-222238 (1996), for example).
However, the interconnector made of lanthanum chromite-based perovskite composite oxide has a large part thereof, except the surface of interconnector in contact with the air, reduced with the fuel gas (e.g., hydrogen) being supplied into the fuel gas passage formed in the support base at the occasion of reduction treatment for activating a catalyst metal or of actually generating power after fired, and the large part of interconnector expands with the support base between the interconnectors, possibly causing the whole cell to deform, damage, etc.
That is to say, lanthanum chromite, which is a chief material of the interconnector, is a p-type semiconductor oxide featuring hole conduction. A method is commonly known of doping an element small in valence number to increase the hole for the purpose of enhancement in conductivity, and such lanthanum chromite exhibits high conductivity in oxidative atmosphere and is stable even at high temperature. However, in reductive atmosphere, oxygen vacancy is generated and the hole is captured by the oxygen vacancy, which situation induces not only a decrease in the conductivity but also a change of Cr in valence number from quadrivant to trivalent, whereby an ion increases in radius and thus causes a crystal lattice to expand, resulting in the volume expansion. This is considered as a mechanism of reduction-induced expansion of the interconnector, and it is suggested that the B site may contain Mg and Ni as dopant causing small volume expansion (JP-A 8-222238 (1996)).
However, according to the above mechanism, lanthanum chromite is reduced to expand more or less, and the reduction-induced expansion is essentially unable to be prevented. This imposes a problem that it is difficult for the fuel cell to ensure its long-term reliability.